Структура электродинамических сил, ускорение плазмы и генерация обратных токов в токовых слоях А.Г. Франк, Н.П. Кирий, С.Н. Сатунин Институт общей физики. - презентация

1 Структура электродинамических сил, ускорение плазмы и генерация обратных токов в токовых слоях А.Г. Франк, Н.П. Кирий, С.Н. Сатунин Институт общей физики им. А.М. Прохорова РАН VI Конференция «Физика плазмы в солнечной системе» в рамках Программы ОФН-15 РАН «Плазменные процессы в солнечной системе» 18 февраля 2011 г.

2 Цели и задачи исследований = Изучение динамики токовых слоев и процессов магнитного пересоединения на основе лабораторных экспериментов позволяют сопоставлять структуру магнитных полей, электрических токов и электродинамических сил, с одной стороны, с параметрами плазменных потоков, которые ускоряются в пределах слоя, с другой стороны. = В лабораторных экспериментах были зарегистрированы направленные движения плазмы со сверхтепловыми скоростями, которые можно, по-видимому, интерпретировать как аналог корональных выбросов массы (CME). Эти исследования дают возможность приблизиться к пониманию физической природы динамических явлений в атмосфере Солнца. Основные задачи данной работы: = Определение пространственно-временных характеристик электрического тока и электродинамических сил на основе анализа магнитных полей токовых слоев, развивающихся в различных условиях; = Регистрация направленных потоков плазмы, которые генерируются в токовых слоях; = Сопоставление направленных скоростей и энергий плазменных потоков с работой сил Ампера; выявление характерных особенностей ускорения плазмы.

3 Schematic of the CS-3D device = 2D magnetic field B = {-h y; -h x; 0} with the null-line at the z-axis, h 1 kG/cm; = Guide field B z aligned with the null line: B z 8 kG; = Superposition of B and B z forms a 3D magnetic configuration with the X line; = Vacuum chamber: quartz, 18 cm, L = 100 cm, is filled with a gas: He, Ar, Kr or Xe; = The initial plasma, N e 0 = cm -3, is produced by -discharge; = Both magnetic fields and the initial plasma are uniform in the z-direction: /z = 0; = Current along the X line: J z 100 kA, T / 2 = 6 s, results in current sheet formation; = Diagnostics: magnetic probes, interference-holography; spectroscopy, X-ray detectors. Cross-sectionSide view

4 Propagation of the magneto-acoustic wave and the in-plane plasma motions in the vicinity of the X line Magnetic field with the X line: B = {-h y; -h x; B Z } Perturbations of the magnetic field propagate as a converging magneto-acoustic wave (MAW) toward the X line in the (x, y) plane. A typical time interval for MAW propagation is defined by the local Alfven velocity: t A = (4 N i M i ) 1/2 / h. Plasma current: j = c /(4 ) rot B. Plasma dynamics is controlled by the Ampere forces: f = 1/c [j B]. Excitation of j Z currents behind the front of MAW brings about plasma compression in the y – direction and the outward motion in the x – direction.

5 Kr, p=36 mTorr; h=0.57 kG/cm; B Z 0 = kG; J Z =70 kA Formation of a current sheet in magnetic field with an X line ( in- plane component B X ) Amplification of the excess guide field B Z A.G. Frank, S.G. Bugrov, V.S. Markov // Phys. Lett. A 373, 1460 (2009)

6 Structure of the magnetic force lines in the (x, y) plane: A Z = const; A Z = 10 3 G cm 2D vacuum magnetic field Ar 20 mTorr; h = 0.64 kG/cm; J Z = 65 kA; t = 1.9 s In-plane magnetic field of the current sheet

7 2D distributions of plasma density at successive time moments t = 2.95 μs t = 3.95 μs t = 4.35 μs h = 0.43 kG/cm; B Z 0 = 2.9 kG; Ar filling, 28 mTorr; J Z max = 50 kA = Formation of a current sheet is accompanied by effective plasma compression into the sheet, with the maximum density 10 times higher than the initial density: N e max cm -3 = Plasma sheet can evolve in the 3D magnetic configuration, in the presence of the strong guide field B Z 0 along the X line. Frank A G et al. Phys. Plasmas (2005)

8 As the temperature increases, Ar +1 and Ar +2 ions become depleted successively turning to higher ionization states. As a result, the spectral lines Ar IV, Ar V, with Ar VI in some cases, should appear in the plasma emission spectrum. These lines, however, fall within a shorter-wavelength UV range (λ < 300 nm). Time evolution of plasma parameters in the sheet midplane: Effective ion charge Z eff Densities of argon ions N i (Ar +1 ÷ Ar +5 ) Electron density N e Electron temperature T e Voronov G.S. et al. Plasma Phys. Rep. 34, 999 (2008) h = 430 G/cm; Ar, 28 mTorr; J z max = 70 kA

9 Current distribution in the (x,y) plane is characterized by 2 different sizes: x / y 6 15 Ar, 20 mTorr; h = 0.64 kG /cm; J Z max = 65 kA; t = 1.9 s In-plane magnetic field components B X, B Y and current density j Z in the current sheet Distributions along the sheet width (x-axis), y = 0.8 cm Distributions along the sheet thickness (y-axis), x = 0.8 cm and x =-5 cm

10 Evolution of the current density j z 0 in the CS midplane and the y-dimensions of CS at the levels 0.5 j z 0 and 0.1 j z 0 Ar, x = 0.8 cm He, x = 0.8 cm He, x = -5 cm Ar, x = -5 cm

11 Scheme of two-channel spectral measurements with the use of a Nanogate 1-UF fast programmable CCD camera Ø z 1.5 cm Ø x 2.5 cm

12 Time behavior of the ion temperature T i and averaged energy of plasma flows W x Ar, 28 mTorr, h = 0.5 kG/cm, J z 75 kA Kyrie N.P. et al. Plasma Phys. Rep.36, 357 (2010) = T i, T e, Z i,av are maximum in the sheet midplane and increase with time; = T i > T e = The plasma is in transverse equilibrium (along the y-axis) with the magnetic field: N e (T e +T i /Z i ) + ( B Z ) 2 /8 B X 2 /8 ; 1

13 The Ampere force F x acting along the current sheet surface = 1.2 cm I z (x) = – c /2 {B x J (x) – [ B y J (x)/ x] }; j z (x) = I z (x) / 2 F x (x) = f x (x) 2 = -1/c I z (x) B y T (x); B y T = h x + B y J h = 0.57 kG/cm B Z 0 = 0 (2D) Ar, 28 mTorr J Z 100 kA; t 1.9 s F x max dynes cm -2

14 Plasma acceleration along the current sheet surface M i N i dv/dt = - p + 1/c [j B] = p is negligible along the current sheet surface (x-direction). = In the 2D magnetic configurations (B z = 0) the Ampere forces f x сome to play only in the presence of the normal magnetic field component B y T : f x = 1/c [j B] x -1/c (j z B y T ) = The average density of the Ampere force f X (x) was calculated on the basis of magnetic measurements: f x (x) -1/c I z (x) B y T (x) / 2 f X (x) dx N i W X eV cm -3 At N i cm -3 W X max 115 eV. = The time interval for accelerating the Ar (+1) ions is 3-5 s. = These estimations correlate with the measured energy of the Ar ions and the typical acceleration time.

15 Comparison between HeII 4686 Å and HeII 3203 Å line profiles observed in the x- and z-direction HeII 4686 Å HeII 3203 Å x-directionz-direction z = 2.4 Å x = 6.0 Å z = 1.6 Å x = 4.6 Å He, 320 mTorr; h = 500 G/cm; B z =0; J z max = 70 kA; t 3 s

16 Тепловые и направленные скорости ионов HeII в токовых слоях, He, 320 mTorr; h = 0.5 kG/cm; B z =0; B z = 2.9 kG J z max = 70 kA N e 0 ( ) cm -3 N e x cm -3 T i 50 eV W x 400 eV (B z = 0) развивающихся в 2D магнитном поле (B z = 0) или в 3D магнитной конфигурации (B z = 2.9 kG) Н.П. Кирий и др. Труды ФАС-XIX, С (2009) SLs: HeII nm; HeII nm

17 The Ampere force F x acting along the surface of a current sheet formed in the He plasma = 1.2 cm I z (x) = – c /2 {B x J (x) – [ B y J (x)/ x] }; j z (x) = I z (x) / 2 F x (x) = f x (x) 2 = -1/c I z (x) B y T (x); B y T = h x + B y J h = 0.5 kG/cm B Z 0 = 0 (2D) He, 320 mTorr J Z 70 kA; t 2.1 s F x max dyn cm -2 x cm

18 The y-dependence of the Ampere force f x (y) at x = -5 cm At the CS midplane (y = 0) there is a maximum in the current density j z (y), and a minimum in the value of the normal component B y T (y). The force f x (y) = -1/c j z (x) B y T (x) can have a local minimum near the midplane. We might expect effective plasma acceleration where plasma density is lower than at the CS midplane, i.e. at some distance along the y – axis. h = 0.63 kG/cm; Ar, 28 mTorr; J Z 70 kA; B Z 0 = 0

19 Ampere force f x (y) and plasma density N e (y) at x = -5 cm He, 320 mTorr h = 0.5 kG/ cm J z max = 70 kA The N e (y) distribution is very narrow as compared with the f x (y) distribution, so that the low-density plasma at wings of the N e (y) distribution can be effectively accelerated

20 Distributions of the current I z (x) at successive times. Development of reverse currents E z i 1/c (v x B y T ) h = 0.63 kG/cm Ar, 28 mTorr J Z 70 kA t = 2.3 s t = 3.5 s t = 4.5 s t = 5.0 s

21 Evolution of the currents I z (x) integrated over one-half the sheet (- R x 0) J z (+) = I z (+) (x) dx - direct currents in the region (x R x 0); J z (-) = I z (-) (x) dx - reverse currents in the region (-R x x R ); J z (S) = I z (x) dx - the total current in the whole region (-R x 0). x R (t) – the x-coordinate where the current I z (x) reverses its direction: I z (x R ) = 0. t, s The current I z (x) is concentrated in the region y = 0.8 cm h = 0.63 kG/cm Ar, 28 mTorr J Z 70 kA

22 Magnetic structure of current sheets, by S.I. Syrovatskii, JETP 1971 A current sheet with the reverse currents at the edges A current sheet without the reverse currents

23 Заключение = В экспериментах по изучению динамики токовых слоев и процессов магнитного пересоединения была исследована эволюция магнитных полей, что позволило определить основные особенности структуры электрических токов и электродинамических сил. = Измерены температуры ионов, электронов и энергии направленных движений плазмы. Обнаружены потоки плазмы, которые движутся вдоль поверхности токового слоя с энергиями, значительно превышающими тепловую энергию ионов. = Проведен анализ пространственной структуры сил [j B] и показано, что под действием этих сил должно происходить постепенное увеличение кинетической энергии направленного движения ионов вдоль поверхности токового слоя. = В результате энергия ионов у боковых концов слоя может достигать 100 эВ, что согласуется с непосредственно измеренными энергиями потоков плазмы при формировании слоя в Ar. = Обнаружено, что у боковых краев слоя возникают токи обратного направления по отношению к основному току, протекающему в центральной области слоя. = = Генерация обратных токов и их усиление со временем свидетельствуют о новых динамических эффектах в токовых слоях, возникающих при движении потоков плазмы в сильном поперечном магнитном поле, что, в свою очередь, приводит к изменению магнитной структуры слоя.

24 Спасибо за внимание!

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27 Coronal Mass Ejections (CME) X-ray images of the Sun recorded with the SPIRIT device mounted on the Coronas-F satellite.

28 Experimental device CS-3D Institute of General Physics, Moscow, Russia 100 cm

29 Distributions over the current sheet thickness of the tangential magnetic field component B X (y), current density j Z (у) and excess guide field B Z (у) Ar, 28 mTorr; h = 0.57 kG/cm; B Z 0 = 4.3 kG; J Z 70 kA = The excess guide field B Z (у) is localized only in the regions where the basic current j Z flows. = The excess guide field B Z is supported by additional plasma currents in the (x, y) plane. = The total current on one side of the current sheet, J X 57 kA, is of the same order as the total basic current along the X line, which gives rise to the current sheet formation, J Z 70 kA.

30 Plasma dynamics in 3D magnetic field with the X-line and the guide field B z Deterioration of the current and plasma compression due to amplification of the guide field in the sheet Compression of the current, plasma and the guide field B z into the sheet

31 h = 430 G/cm; Ar, 28 mTorr; J z max = 70 kA Voronov G.S. et al. Plasma Phys. Rep. 34, 999 (2008) Ar +1 and Ar +2 ions are depleted in the sheet midplane with increasing T e and N e T e was determined from time behaviour of various spectral lines by using the SIMPTOS code including the processes of ionization, excitation and plasma flows. Spatiotemporal evolution of plasma parameters under study: Intensity of spectral line Ar II nm (Ar +1 ions) Intensity of spectral line Ar III nm (Ar +2 ions) Electron density N e and electron temperature T e